The objective of the proposed research is to establish the cellular basis of active convergence (narrowing) and extension (lengthening) that occurs in early development of the amphibian, Xenopus laevis. This morphogenetic movement, convergent extension, is the major force-generating process in gastrulation and early notochord formation in amphibians and also occurs in fish, birds, mammals, and many invertebrates. In vertebrates, it is important in organizing the dorsal-ventral polarity of the animal, in the development of the nervous system, and in the subsequent events of epigenesis. Despite its importance, this morphogenetic movement is poorly described and its cellular basis is unknown. Preliminary evidence shows that in amphibians convergent extension occurs by intercalation of cells to form an array that is fewer cells in width and more in length. My aims in the proposed research are, firstly, to determine what pattern of intercalation occurs in the intact gastrula or in explants of the gastrula by monitoring the intercalation of cells labeled with a fluorescent cell lineage tracer with adjacent unlabeled cells. Secondly, I will use time-lapse video and cinemicrography to record directly, the cellular motile behavior that brings about intercalation, using a new culture medium and culture system that allows such direct observation of morphogenetic events previously hidden from view in the gastrula. Thirdly, I will show what details of morphology and ultrastructure are associated with specific behavioral events by fixing the cells during intercalation and processing identified groups of cells for electron microscopy. Fourthly, I will analyze the effect of several experimental manipulations, known to ffect convergent extension, on the detailed behavior of intercalating cells. Lastly, I will develop experimental conditions and methods necessary for analysis of the cytoskeleton and extracellular matrix involved in cell intercalation. The results will establish a cellular basis for this important morphogenetic process and prepare for molecular analyses to follow.